when deploy registry as a pull through cache ,function copeContent() may cause a socket leak when docker user canceled its pull operation.

Signed-off-by: yaoyao.xyy <yaoyao.xyy@alibaba-inc.com>

registry/proxy/proxyblobstore.go

@@ -55,6 +55,8 @@ func (pbs *proxyBlobStore) copyContent(ctx context.Context, dgst digest.Digest,
 		return distribution.Descriptor{}, err
 	}
 
+	defer remoteReader.Close()
+
 	_, err = io.CopyN(writer, remoteReader, desc.Size)
 	if err != nil {
 		return distribution.Descriptor{}, err